Implementing condition_variable timed_wait correctly

The trick is to use a pthread_condattr_setclock to tell the pthread_condattr_t to use CLOCK_MONOTONIC. The C code for doing this is pretty simple:

#include <time.h>#include <pthread.h>#include <errno.h>#include <stdio.h>int main(){    // Set the clock to be CLOCK_MONOTONIC    pthread_condattr_t attr;    pthread_condattr_init(&attr);    if (int err = pthread_condattr_setclock(&attr, CLOCK_MONOTONIC))    {        printf("Error setting clock: %d\n", err);    }    // Now we can initialize the pthreads objects with that condattr    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;    pthread_cond_t  cond;    pthread_cond_init(&cond, &attr);    // when getting the time, we must poll from CLOCK_MONOTONIC    struct timespec timeout;    struct timespec now;    clock_gettime(CLOCK_MONOTONIC, &now);    timeout.tv_sec = now.tv_sec + 5;    timeout.tv_nsec = now.tv_nsec;    // business as usual...    pthread_mutex_lock(&mutex);    int rc = pthread_cond_timedwait(&cond, &mutex, &timeout);    if (rc == ETIMEDOUT)        printf("Success!\n");    else        printf("Got return that wasn't timeout: %d\n", rc);    pthread_mutex_unlock(&mutex);    return 0;}

I'm going to leave this open for a while because somebody might have an easier answer. The thing I'm not happy about here is that it means a wait_until is rather difficult to implement with a real-time clock (my best solution to that is to convert the provided Clock in the time_point into the steady_clock's time and go from there...it is still subject to time-change race conditions, but if you're specifying a timeout in real time, you're already making a terrible mistake).